Special pairs dimer

A dimer made up of two zinc porphyrins bearing a 7-azabicy-clo[2.2.1]heptadiene fused at the C2-C3 /3-positions was reported by Knapp (61). The compound was designed to dimerize with a pyrrole-over-pyrrole geometry similar to that found in the photosynthetic special pair. Dimerization at KT3 M was confirmed by VPO and JH NMR spectroscopy. Dilution to 10-5 m or addition of DMAP caused disaggregation of the complex. In the solid state, this compound assembles as a cyclic hexamer with the vicinal porphyrin planes almost perpendicular. 

This is proof that the signal at 24 MHz is due to a transition within an excited electronic state involving the special pair dimer. 

THEORETICAL STUDIES ON THE ELECTRONICAL STRUCTURE OF THE SPECIAL PAIR DIMER AND THE CHARGE SEPARATION PROCESS FOR THE REACnON CENTER RHODOPSEUDOMONAS VIRIDIS... 

The spectroscopy and dynamics of photosynthetic bacterial reaction centers have attracted considerable experimental attention [1-52]. In particular, application of spectroscopic techniques to RCs has revealed the optical features of the molecular systems. For example, the absorption spectra of Rb. Sphaeroides R26 RCs at 77 K and room temperature are shown in Fig. 2 [42]. One can see from Fig. 2 that the absorption spectra present three broad bands in the region of 714—952 nm. These bands have conventionally been assigned to the Qy electronic transitions of the P (870 nm), B (800 nm), and H (870 nm) components of RCs. By considering that the special pair P can be regarded as a dimer of two... 

We calculate three models (1) the dimer model presented in this work (2) the dimer model employed by Scherer et al. and (3) the delocalized model. Table V lists the calculated results as a function of temperature. The anisotropy values for r(ei,e2) at 295 K are found to be quite different between the dimer and delocalized models. The difference is about 53%. Meanwhile, the differences at other temperatures are within about 13-14%. For r(e2,23), the differences are within 24-28%. Table Valso lists the angle between B and B2 in the special pair of R26.Phe-a RCs as a function of temperature. 

The primary donor in Photosystem I P700 is thought to be a special pair of chlorophyll a molecules. Katz and Hindman (18) have reviewed a number of systems designed to mimic the properties of P700 ranging from chlorophyll a in certain solvents under special conditions where dimers form spontaneously (19) to covalently linked chlorophylls (20). Using these models it has been possible to mimic many of the optical, EPR and redox properties of the in vivo P700 entity. 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Madjet Mel A, Muh F, Renger T (2009) Deciphering the influence of short-range electronic couplings on optical properties of molecular dimers application to special pairs in photosynthesis. J Phys Chem B 113 12603-14... 

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

The data on the photoexcited triplets are a potential source of structural information. Specifically, dimerization effects on zfs s and kinetics may be of aid in the characterization of special pair... 

The results of EPR studies of photoezcited triplets of model systems show that it is not possible to give generally applicable rules for the interpretation of the spectroscopic data. In a number of cases there appears to be a well-understood relationship between dimerization effects and dimer geometry. In most of the systems considered here that is not the case. It is not clear to what difference in make-up of the dimers this discrepancy must be attributed and this is an interesting point of further investigation. Evidently, as long as the data on fairly well characterized model systems are not fully understood it will be impossible to derive definitive conclusions concerning the structure of the special pair from data on its photoexcited triplet state. 

Spectral measurements suggesting exciton splitting were among early observations that led to the conclusion that the bacteriochlorophyll involved in the initial photochemical process exists as a dimer or special pair (Fig. 23-31),319/324 a conclusion verified by the structure determination. The special pair of BChl b lies in the center of the helical bundle that is embedded in the membrane. Nearly perpendicular to the rings of the special pair are two more molecules of BChl b. The central magnesium atoms of all four bacteriochlorophylls are held by imidazole groups of histidine side chains.319 325 Below the chlorophylls are... 

Natural photosynthesis provides the most dramatic demonstration of the potential hidden in this basic photoreaction. In (bacterial) photosynthesis a chlorophyll-dimer (BC)2—the special pair —receives the radiation energy and thereby gains the energy required to enable it to transfer an electron to a pheophytin moiety (BP), an act occurring within 2-3 picoseconds (Martin et al. 1986) even at very low temperatures. Subsequently the electron is transferred to a quinone acceptor (MQ), which once again occurs (Holten et al. 1978) on a very short time scale of about 230 ps. 

Cowan, J.A., Sanders, J.K.M., Beddard, G.S. and Harrison, R.J. 1987. Modelling the photosynthetic reaction centre photoinduced electron transfer in a pyromellitimide-bridged special pair Porphyrin Dimer, J. Chem. Soc., Chem. Commun., 55-58. 

A view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

The bacteriochlorophyll dimer (special pair), accessory bacteriochlorophyll, and bacteriopheo-phytin are denoted, respectively, as P,B, and H. 

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